1. Field of the Invention
The present invention relates to a communications device, particularly but not exclusively, to controlling the frequency of a generating source in response to perturbations in the supply voltage due to significant levels of noise. The present invention also relates to a frequency generating circuit and to an integrated circuit including the frequency generating circuit.
2. Description of the Related Art
The communications device may be a one-way pager having a receiver stage including a local oscillator source or a two-way device such as a two-way pager, a cellular telephone and a cordless telephone equipped with a transceiver with a frequency generator for generating the local oscillator frequency for the receiver and the master oscillator frequency for the transmitter. These devices include continuously running circuits which introduce significant levels of noise on the dc supply voltage. This problem is particularly relevant for battery powered devices operating at voltages of the order of 1 volt.
U.S. Pat. No. 5,101,178 discloses a crystal oscillator for a battery powered portable telephone. In order to compensate for changes in frequency in response to changes in the dc source voltage, the source voltage is compared against a reference voltage and a control signal is produced on the basis of whether the source voltage is above or below the value of the reference voltage, a capacitor is added/not added to compensate or not for any change in oscillator frequency which is intended to be a fixed frequency. A drawback to this known circuit is that the frequency compensation is not continuous because the addition of, or disconnection of, the capacitor produces a step-like change in the oscillator frequency. Also it is not intended to deal with the effects of perturbations on the dc voltage source due to noise generated by other circuits within the device.